The speakers located in dash and in the rear of an extended cab model will be 4x6 speakers. These speakers are used up until the 94 model.
6.5"
The standard speaker size on a 1996 GMC Sierra truck is 9 inches. Larger speakers can be added but required the opening to be enlarged.
4x6 inches
The speakers in a 2006 GMC Sierra 1500 crew cab are 6.5 inches. However, the speaker manufacturer will refer to this size as 6.75 or 6 3/4 inch speakers.
1988
The 2009 GMC Sierra-3500HD has a V8 engine.
The 2008 GMC Sierra-2500HD has a V8 engine.
The 2010 GMC Sierra-1500 has a V6 engine.
The 2002 GMC Sierra-2500HD has a V8 engine.
The 2007 GMC Sierra-2500HD has a V8 engine.
The 2011 GMC Sierra-1500 has a V6 engine.
The 2011 GMC Sierra-3500HD has a V8 engine.